1. Field of the Invention
The invention relates to adhesive compositions useful as tie-layers for coextruded barrier films. The adhesive compositions which provide improved clarity are blends of an ethylene-α-olefin copolymer base resin having hard and soft phases that form a network structure and an ethylene-α-olefin copolymer grafted with an ethylenically unsaturated carboxylic acid or carboxylic acid derivative. The invention also relates to multi-layer barrier film constructions having improved clarity obtained using the improved adhesive blends.
2. Description of the Prior Art
Multi-layer films are widely used in the packaging industry. Depending on the intended end-use application of the film, the number and arrangement of the various layers as well as the types of polymer resins employed will vary. Polymer resins used include a wide variety of polar and non-polar polymers selected for their ability to provide essential properties such as structural integrity, puncture resistance, abrasion resistance, etc. Resins having barrier properties are generally included as a barrier layer to prevent the passage of oxygen and moisture in multi-layer constructions used for food packaging. Commonly used resins for these applications include polyethylenes, ethylene-vinyl acetate (EVA) copolymers, ethylene-vinyl alcohol (EVOH) copolymers, nylons and the like.
To facilitate adhesion of layers of dissimilar polymer resins, adhesive layers referred to as tie-layers, are employed. The tie-layers are generally mixtures of functionalized polyolefins, i.e., having polar functionality grafted onto the polymer backbone, and non-functionalized polyolefins. While various tie-resin compositions are known to effectively adhere polyethylene to EVOH or nylon this is often accompanied by an undesirable decrease in film clarity generally believed to be the result of interfacial distortion caused by the reaction of the functionality of the tie resin with the functional groups of the EVOH or nylon barrier resin. While the adhesive bond strength is directly proportional to the number of bonds formed at the tie-EVOH (or nylon) interface, this same bonding effectively amplifies shear stresses caused by viscosity differences between the tie resin and barrier resin. The phenomena can be observed by coextruding linear low density polyethylene (LLDPE) and EVOH film and adding increasing amounts of grafted maleic anhydride (MAH) to the LLDPE. As the amount of the grafted MAH increases, there is a corresponding increase in adhesion and interfacial distortion, i.e., reduction in film, clarity. This occurs even though the viscosity of the LLDPE does not significantly change as the grafted MAH is added. The degree of interfacial distortion is determined using Narrow Angle Light Scattering techniques (ASTM D1746-03).
Ethylene copolymers having a network structure are disclosed in U.S. Pat. No. 6,171,993. U.S. Pat. No. 6,177,516 discloses adhesive blends containing 35 to 95 weight percent ethylene copolymer having a network structure and 5 to 65 weight percent modified polyolefin containing acid or acid derivative functionality. The adhesive blends, which preferably additionally contain 7 to 35 weight percent of an elastomeric homopolymer or copolymer, are primarily utilized as tie-layers to adhere multi-layer constructions subjected to high stress/high demand conditions where it is observed the compositions retain their adhesive strength and resist delamination for extended periods.
It would be highly advantageous if adhesive compositions suitable for use as tie-layers to bond barrier polymers, such as nylon and EVOH, and which provide improved clarity were available. It would be even more advantageous if the adhesive compositions exhibited improved clarity in coextruded barrier films while maintaining acceptable adhesion. These and other advantages are obtained with the compositions of the invention which are described in more detail to follow.